Continuous non-sampling measurement of arterial PO2 is desirable in some types of physiological investigations and some acute clinical situations. If the skin can be adequately perfused, oxygen from arterial blood will reach the surface in measurable quantities. This transcutaneous oxygen measurement coupled with a measure of relative skin blood flow, may provide a good measure of arterial PO2. This program is designed to develop a suitable oxygen electrode and perfusion measuring instrument for transcutaneous use.